Nickel aluminide coated low alloy stainless steel

ABSTRACT

Low alloy austenitic stainless steel is protected from oxidative corrosion and erosion at high temperatures in the presence of exhaust gases from internal combustion engines by flame-spraying a layer between about 0.003 inches and about 0.007 inches thick of an alloy of 80 percent nickel and 20 percent chromium on the steel base, and thereafter converting the layer to an intermetallic chromium containing nickel aluminide coating by a low temperature aluminum cementation process. A cementation pack comprising 96 percent aluminum oxide, 2 percent aluminum, 1 percent sodium chloride, and 1 percent ammonium bifluoride is placed on the nickel-chromium layer in an inert atmosphere for a period between four hours and 24 hours at 1400*F to 1600*F to form an intermetallic chromium containing nickel aluminide coating containing 40 to 60 atomic percent aluminum. The coated steel is highly resistant to oxidation corrosion and erosion by high temperature exhaust gases from internal combustion engines. It is particularly useful in the construction of thermal reactors for exhaust systems in automobiles for the control of pollution from exhausts.

United States Patent 1 Grisaffe et al.

[ 1 Oct. 2, 1973 [75] Inventors: Salvatore J. Grisaife, Rocky River;

Ernest W. Klechke, Fairview Park, both of Ohio [73] Assignee: The UnitedStates of America as represented by the National Aeronautics and SpaceAdministration, Washington, DC.

22 Filed: Oct. 18, 1971 211 Appl. No.: 190,316

3,505,028 4/l970 Douthit 1. 29/1962 3,597,l72 8/]971 Blingardt 29/l96.23,640,755 2/1972 Barth 29/1962 3,653,852 4/l972 Seller 1. 29/196!)Primary Examiner-Hyland Bizot Attorney-N. T. Musial et al.

[57] ABSTRACT Low alloy austenitic stainless steel is protected fromoxidative corrosion and erosion at high temperatures in the presence ofexhaust gases from internal combustion engines by flame-spraying a layerbetween about 0.003 inches and about 0.007 inches thick of an alloy of80 percent nickel and 20 percent chromium on the steel base, andthereafter converting the layer to an intermetallic chromium containingnickel aluminide coating by a low temperature aluminum cementationprocess. A cementation pack comprising 96 percent aluminum oxidc, 2percent aluminum, l percent sodium chloride, and 1 percent ammoniumbifluoride is placed on the nickel-chromium layer in an inert atmospherefor a period between four hours and 24 hours at l400F to 1600F to forman intermetallic chromium containing nickel aluminide coating containing40 to 60 atomic percent aluminum.

The coated steel is highly resistant to oxidation corrosion and erosionby high temperature exhaust gases from internal combustion engines. Itis particularly useful in the construction of thermal reactors'forexhaust systems in automobiles for the control of pollution fromexhausts.

6 Claims, No Drawings NICKEL ALUMINIDE COATED LOW ALLOY STAINLESS STEELORIGIN OF THE INVENTION The invention described herein was made byemployees of the United States Government and may be manufactured andused by or for the Government for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Field of the Invention This inventionrelates to a coating for low alloy austenitic stainless steel to renderthe steel highly resistant to oxidation corrosion and erosion fromexhaust gases in internal combustion engines.

The engine exhaust of automotive internal combustion engines containscarbon monoxide, carbon dioxide, nitrogen dioxide, unburnedhydrocarbons, sulfur compounds, and other corrosive products which areexhausted into the atmosphere in the absence of means for controllingsuch emissions. Recently federal regulations have been imposed which setprogressively lower allowable engine exhaust levels for such combustionproducts. Minor adjustments in carburetion and spark control and thelike are no longer satisfactory in meeting such requirements. One waysuch engine emissions can be reduced further in the future in order tomeet the requirements of controlling agencies is by the utilization ofthermal exhaust converters or reactors. Such thermal reactors replacethe exhaust manifold.

In some reactors, secondary air is introduced into the reactor in orderto promote the complete combustion of unburned hydrocarbons and carbonmonoxide. In order to achieve the desired ends, many components of thethermal reactor must operate at temperatures on the order of 1900F withthe possibility that if the ignition means fails, the temperature mayincrease up to 2300F.

It is a practical requirement of the automobile industry that suchreactors have useful lives of 2000 hours or more, which translates to50,000 miles of driving at an average speed of 25 miles per hour. Undersuch stringent conditions of severe temperatures, exhaust atmosphere,and term of life requirements, the oxidation corrosion and erosion aswell as the chemical attack by lead and sulfur compounds on unprotectedsteel components are very significant.

Since such thermal exhaust reactors are required in large quantities,for example, up to fifteen million units a year at the presentautomotive industry production rate, it is essential that such thermalreactors be capable of being produced at the minimum possible cost. Oneapproach to avoid utilization of expensive components is to useprotective coatings on the metal materi als used in such thermalreactors.

DESCRIPTION OF THE PRIOR ART It is known, of course, that materials suchas extremely high alloy steels are resistant to oxidation corrosion anderosion and thus fulfill the technical requirements for materials insuch thermal exhaust reactors. The use of high alloy steels would beextremely expen- 'sive for such applications, however, because of thevast amounts of metal such as nickel that are required. It is known thatlow alloy steels possess moderately high temperature strengths but thatthey are not resistant to degradation by the environmental conditions inthe thermal exhaust reactors.

In US. Pat. No. 3,481,715, Whalen et al., there is disclosed a sealingmember which is moderately resistant to oxidation at high temperaturesin which the steel base is coated with an intermediate layer of analloy-of nickel with aluminum or chromium appliep by a plasma gun, towhich a surface layer of material comprising nickel oxide and calciumfluoride is applied-using a plasma gun. The nickel with aluminum orchromium layer serves to improve the adhesion of the oxide surfacelayer.

U.S. Pat. No. 3,338,733, Rowady, discloses metallic surface coatingsresistant to corrosion and wear such as that to which internalcombustion valves are exposed. In this case a steel base is firstpreheated to 1300 to l800F and a nickel-chromium alloy is applied byflame-spraying. The coated object is then subjected to a first diffusionstep at temperatures of l800 to 2250F to promote alloying between thecoated alloy and the base metal. The coating subsequently may be coatedwith an aluminum layer by first preheating the coated base to l200 tol400F and then thereafter dipping the article in a molten bath ofaluminum or applying the aluminum by a flame-spraying technique. Afterthe aluminum coating has been applied, the object is then heated to1500F to 2l00F to diffuse and alloy the aluminum into thenickel-chromium alloy.

SUMMARY OF THE INVENTION The present invention provides a low alloyaustenitic stainless steel article highly resistant to oxidationcorrosion and erosion by a coating which comprises a chromium containingnickel aluminid-e applied by the process of this invention. Low alloystainless steel material I coated according to this invention exhibitsoxidation resistance equal to or better than the oxidation resistanceobtained on many highly allloyed steels either uncoated, or coated withexpensive alloys and other compounds, many applied by complex processes.

A relatively low cost stock article suitable for use in constructingthermal exhaust reactors is provided by this invention by using lowalloy austenitic stainless steel. It provides resistance to corrosionusing steels having only a relatively small amount of nickel.

According to this invention, an oxidation corrosion and erosionresistant article is provided by depositing on a low nickel alloyaustenitic stainless steel basea layer between about 0.003 to 0.007inch, preferably 0.005 inch, thick ofa nickel-chromium alloy such as 80percent nickel-20 percent chromium, by means of flame-spraying or plasmaspraying, then converting the nickel-chromium alloy layer to onecomprising an intermetallic compound, as opposed to an alloy. ofchromium-containing nickel aluminide by a low tem' perature aluminumpack cemcntation process at l400 l600F for four to 24 hours which in asingle process step also serves to diffusion bond the coating to thesteel substrate. The chromium-containing nickel aluminide coatingcontains from about 40 to about atomic percent of aluminum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Test specimens of austenitic304 stainless steel (Fel8Cr8Ni0.008C) were cut from 1/16 inch thicksheet stock. One set were inch X Ht: inches, and an other were 1 inch X2 inches. Prior to coating according to this invention the specimenswere ball milled in a ceramic mill with no. 36 alumina grit and waterand thereafter vapor blasted with l mesh alumina, degreased, and thenultrasonically cleaned in trichloroethylene.

The samples are then coated with a 0.005 inch layer of 80 percentnickel-20 percent chromium by flamespraying, using a conventional spraygun adapted for spraying nickel-chromium alloy fed in wire form. Thethickness of the layer can be between 0.003 and 0.007 inches. Layershaving thicknesses at the lower end of the range have less stresseswhile the thicker layers have longer lives.

The nickel-chromium layer is then converted to a chromium containingnickel aluminide intermetallic compound by a pack cementation step bytreating the layer with a high activity aluminum pack. The pack employedcomprises 96 percent aluminum oxide, 2 percent aluminum, 1 percentsodium chloride, and l percent ammonium bifluoride. The coating istreated in a furnace under an inert atmosphere such as argon for fromfour to 24 hours, preferably for 16 hours, at l600F. The processdeposits approximately mg. of aluminum per square centimeter whichreacts to form a chromium enriched nickel aluminide intermetallicprotective layer on the low alloy austenitic stainless steel and at thesame time forms a diffusion bond between the coating and the substrate.

Essentially no diffusion bonding takes place during the flame-sprayingapplication of the nickel-chromium layer. The bond between the alloylayer and the steel base at this stage is itself primarily a mechanicalbond. Following the aluminum cementation, the chromiumnickel alloy layerhas been converted to a chromiumcontaining nickel aluminideintermetallic compound coating which is diffusion bonded to the steelsubstrate.

The inch X 1% inches samples are then placed in a horizontal multitubefurnace on high purity recrystallized alumina boats. Air is passed overthe specimens at the rate of two cubic feet per hour. One set ofspecimens is heated for 100 hours at 2000F, with cycling to roomtemperature. The cycles consist of hours at test temperature followed bycooling to room temperature. At room temperature the specimens arelightly brushed to remove any oxide scale and weighed to 0.05 mg. persquare centimeter accuracy. A second set of specimens is similarlycycled for 600 hours at l800F. Excellent resistance to oxidationcorrosion and erosion was evident from visual examination, x-raydiffraction, microscopic and metallographic analysis. Furthermore, therewas no loss of weight of the samples, but a slight weight gain, forexample, the specimens gain about 3 mg. per square centimeter after 600hours exposure to the l800F cycling tests. During all of the tests, themetal temperature is measured by thermocouples which are mounted onadjacent uncoated specimens.

The 1 inch X 2 inches coated specimens are tested in an experimentalautomotive thermal reactor connected to a high performance 472 cubicinch V-8 automobile engine with air injection and carburetion. Leadedgasoline is used as a fuel and the engine is operated to provide testspecimen cycles of IO minutes at l900F, 5 minutes at 1200F, and repeat.After l70 cycles, the specimens exhibit no significant deterioration andgain approximately 0.l gram. All coatings after testing exhibitexcellent weight stability, coating preservation and metallographicstability. The specimens when examined visually appear almost unchangedfrom the as coated condition.

The coatings obtained according to the process ofthe present inventionimpart excellent high temperature corrosion and erosion resistance tolow alloy austenitic stainless steel. The protection of the low alloyaustenitic stainless steel is achieved by a process which involvesprimarily only two steps as opposed to other methods of applyingdifferent oxidation resistant coatings which are often quite complex.The invention is particularly advantageous in providing an articlesuitable for utilization in automotive thermal reactors which arenecessary to reduce exhaust pollution in automobiles. Significantly thepresent invention provides an excellent material for such reactors atrelatively low cost and a low consumption of expensive nickel metal. Byway of illustration, in order to provide 15 million thermal exhaustreactors annually utilizing a high alloy stainless steel such asaluminum coated lnco 800 containing 32 percent nickel, about 15 percentof the total nickel consumed annually in the United States would berequired, whereas utilizing material according'to the present inventionabout 7 percent of the nickel consumed annually would be required. Thisis based upon the use of 4% square feet of material 1.5 mm. thick for atypical thermal core reactor.

While the invention has been explained by a detailed description of aspecific embodiment, it is understood that various modifications andsubstitutions can be made within the scope of the appended claims whichare intended to include equivalents of such embodiments.

What is claimed is:

1. An article of manufacture resistant to oxidative corrosion anderosion at high temperatures comprising an austenitic stainless steelbase having a nickelchromium alloy coating with a thickness of about0.005 inch forming a diffused modified bonded junction with thestainless steel base, said alloy coating having aluminum diffusedthercinto to form nickel aluminide which increases from the base surfaceto the outer surface so that said outer surface comprises substantiallyall nickel aluminide and chromium.

2. An article of manufacture as claimed in claim 1 wherein said coatingis produced by applying aluminum by means of low temperature packcementation to a flame-sprayed layer between about 0.003 inch and about0.007 inch thick of percent nickel-20 percent chromium alloy on saidbase, said'aluminum reacting with said layer to form said nickelaluminide.

3. An article of manufacture as claimed in claim 2 wherein saidaustenitic stainless steel is a low nickel alloy stainless steel, andsaid aluminum is applied from a cementation pack comprising 96 percentaluminum oxide, 2 percent aluminum, 1 percent sodium chloride, and 1percent ammonium bifluoride.

4. An article of manufacture as claimed in claim 2 wherein said aluminumis applied in an inert atmosphere at about l400 to 1600F.

5. An austenitic stainless steel substrate having a flame sprayed alloycoating of 80 percent nickel and 20 percent chromium with a minimumthickness of 0.003 inch, said alloy coating having aluminum diffusedthercinto to form nickel aluminide which increases from the stainlesssteel surface to the outer surface thereof so that said outer surfacecomprises substantially all nickel aluminide and chromium, said coatingforming a diffused modified bonded junction with the stainless steelsubstrate.

6. An article as claimed in claim 5 wherein said flame-sprayed layer isabout 0.005 inch thick.

2. An article of manufacture as claimed in claim 1 wherein said coatingis produced by applying aluminum by means of low temperature packcementation to a flame-sprayed layer between about 0.003 inch and about0.007 inch thick of 80 percent nickel-20 percent chromium alloy on saidbase, said aluminum reacting with said layer to form said nickelaluminide.
 3. An article of manufacture as claimed in claim 2 whereinsaid austenitic stainless steel is a low nickel alloy stainless steel,and said aluminum is applied from a cementation pack comprising 96percent aluminum oxide, 2 percent aluminum, 1 percent sodium chloride,and 1 percent ammonium bifluoride.
 4. An article of manufacture asclaimed in claim 2 wherein said aluminum is applied in an inertatmosphere at about 1400* to 1600*F.
 5. An austenitic stainless steelsubstrate having a flame sprayed alloy coating of 80 percent nickel and20 percent chromium with a minimum thickness of 0.003 inch, said alloycoating having aluminum diffused thereinto to form nickel aluminidewhich increases from the stainless steel surface to the outer surfacethereof so that said outer surface comprises substantially all nickelaluminide and chromium, said coating forming a diffused modified bondedjunction with the stainless steel substrate.
 6. An article as claimed inclaim 5 wherein said flame-sprayed layer is about 0.005 inch thick.